It is only the latest surprise. Most forecasters expected the Bank of England’s unprecedented money-printing program, in which it bought government bonds equivalent to 20% of gross domestic product, would trigger a recovery. Yet output remains 3% below its peak in late 2007.

Most forecasters expected the 20% depreciation in the pound since the start of the crisis to spark a recovery in exports. It didn’t happen.

Economic theory suggested the faltering economy would stoke unemployment, which in turn would cause output per worker to rise. Yet unemployment only rose modestly to 8% while productivity is still 4.4% below pre-crisis levels.

Economic theory also suggested that a weak economy would mean lower inflation. Yet since 2007, it has averaged nearly a percentage point above the BOE’s 2% target and well above rates in other European countries.

Now the U.K. is providing another mystery. This week’s GDP data are likely to show that the economy grew 0.8% in the third quarter compared with the previous quarter. Indeed, if recent survey data are to be believed, growth could be heading for 5% year-over-year, according to Rob Wood, chief U.K. economist at Berenberg Bank.

Yet this growth spurt has taken economists almost entirely by surprise. It is hard to identify a single leading forecaster predicting such a strong recovery at the start of the year, or even at the end of the first quarter.